1. Field of the Invention
The present invention relates to a developing method in which an optical signal is radiated onto a recording region from the back surface of an image carrying element made of a photo-semiconductive material having light-transmitting properties, so that toner in a magnetic developer conveyed by a developing roll comprising a permanent magnet and a sleeve is selectively adhered onto the image carrying element.
2. Description of the Related Art
Means for electrophotography, electrostatic recording or electrostatic printing have generally been used as means of reproducing information such as documents or figures. In electrophotographic copying machines, facsimile equipment or the like, the following image forming method has often been employed: an electrostatic latent image is formed on a photo-conductive layer or a dielectric layer; and then, a toner is selectively adhered to the latent image by a magnetic brush method (the toner comprising a toner charged in a predetermined polarity by triboelectric charging due to the contact of the toner particles with carrier particles, or a magnetic toner mainly containing a binder resin and a magnetic powder), thereby obtaining a reproduced image. The adoption of such an image forming method, however, entails some problems. For example, electrostatic latent image forming means including charging means for uniformly charging the image carrying element beforehand has to be provided in addition to the developing means in the image forming apparatus. As a result, the apparatus may become complicated and large.
In order to cope with the problems, the following has been proposed, for example, in U.S. Pat. No. 3,816,840: a magnetic toner having electrically conductive properties is magnetically attracted and held onto a sleeve made of a non-magnetic, electrically-conductive material; and an information signal is applied to a recording electrode opposed to the sleeve while a sheet-shaped recording medium is passed through the gap between the sleeve and the recording electrode, so as to impart a force of static electricity to magnetic toner, which is selectively adhered to the recording medium.
Various proposals, including the above, have been made concerning direct recording means. In each proposal, recording is effected by an electric means only while a recording medium is passed through the gap between a recording electrode and an opposing electrode. Accordingly, in order to obtain good recorded images, it is necessary to strictly set various conditions such as the size of the gap between the electrodes and the amount of toner supplied to a recording medium. If the recording medium comprises normal paper, this entails certain problems. Since the surface resistivity of the paper is greatly influenced by changes in surrounding conditions such as humidity and temperature, the development conditions have to be adjusted in accordance with such surrounding conditions, and, in addition, it is impossible to effect high-speed recording. Therefore, the use of normal paper has not been put into practice.
Another image forming method has been proposed. In this method, an optical signal corresponding to an original image is radiated from the back surface of an image carrying element made of a photo-semiconductive material having light-transmitting properties. An electrically-conductive, magnetic toner which has been conveyed by a developing roll comprising a permanent magnet and a sleeve, is selectively adhered to the surface of the image carrying element. The thus reproduced image is transferred to the surface of a recording medium, and then, fixed.
Such an image forming method including backsurface exposure uses a magnetic toner having a volume resistivity of, for example, from 10.sup.4 to 10.sup.12 .OMEGA..multidot.cm (i.e., within the so-called medium resistivity range). Although the method has good development characteristics, it has a problem in that the efficiency of transferring reproduced images is low. That is, even when a corotron system, the most common type of transfer means, is used, a reproduced image cannot be completely transferred to the surface of the recording medium, thereby causing a phenomenon known as "transfer blur", which results in deteriorated image quality. Thus, the method is disadvantageous in that normal paper cannot be used as a recording medium.
In order to improve transfer efficiency, it is possible to uniformly charge a recording medium beforehand, however, this arrangement makes the entire image forming apparatus complicated, and is not preferable because it thus makes it impossible to achieve smaller sizes and lower prices which have recently been required from apparatuses of this kind. Another possible means of improving transfer efficiency comprises using recording paper which has already been surface-treated. However, this arrangement increases the paper cost, and, in addition, is not fit for apparatuses of the kind being discussed which are to process and record a large amount of information. Thus, it is desired that a developing method capable of providing high transfer efficiency and capable of forming high-quality reproduced images even when normal paper is used, be realized.